The pilot reported that, during a maintenance test flight, when the airplane reached rotation speed on takeoff, he applied elevator control to rotate. The elevator control could not be moved sufficiently aft for rotation and liftoff, and the pilot aborted the takeoff. During the aborted takeoff, the airplane travelled off the departure end of the runway and came to a stop on an airport access road. The airplane's left main landing gear collapsed and the left wing was bent during the accident. The pilot reported that he had performed a control system check prior to the flight and that the controls functioned normally during that check. Examination of the airplane revealed that the elevator control operated normally after the accident, but further examination revealed that a plastic elbow fitting for the windshield defroster duct was loose and could be rotated into a position that obstructed full motion of the control yoke. Examination of the elbow revealed that it was not an approved part and appeared to be the type of plastic elbow found in home supply stores. The recent maintenance performed was not in the area where the plastic elbow was located.